Realization of the triple point of carbon dioxide in a transportable cell using long-stem SPRTs

Abstract

The action of Minamata Convention requests in the near term to develop alternative fixed points to replace the mercury triple point (Hg TP) from the set of the defining points of the International Temperature Scale of 1990. The carbon dioxide (CO2) TP is one candidate that inherits the advantage of high chemical, physical stability and low health risk, and the disadvantage of a relative high pressure in room temperature. We report our study on the realization of the CO2 TP using long stem SPRTs. We present a novel design for CO2 TP cell of the thermometer immersion depth extending from 110 mm to 200 mm with no increase of sample gas pressure. The outer dimension of the new cell is proper for a drop-in compatible replacement of a normal cell for the Hg TP. We realized 14 plateaus of the CO2 TP using two long stem SPRTs in a span of 12 months. The mean temperature at the CO2 TP is 216.59130 (36) K at the melted fraction F=0.5 and 216.59136 (37) K at F=1.0. In the given uncertainties, the uncertainty arising from the transformation to ITS-90 contributes a major part of 0.29 mK to 0.30 mK. The uncertainty formed by the pressure head effect and the axial heat leak through thermometer well of the new cell, the impurities of the sample gas, the inaccuracy of the electronic instruments contribute a less part of 0.21 mK to 0.22 mK. The plateaus consistently exhibit small slopes of the height of less than 0.21 mK and the time spans from 147 hours to 347 hours between F=0.3 and F=0.75. The small slopes in the plateaus demonstrate the repeatability of 0.056 mK and 0.059 mK respectively for the melting fraction F=0.5 and 1.0, the long term stability of 0.031 mK.